In the field of an optical pickup apparatus in recent years, there has been an increasing tendency toward the use of short wavelength for the laser light source used as a light source for reproduction of the information recorded on an optical disc and for recording of information on the optical disc. For example, there have been efforts made for commercial use of a laser light source having a wavelength of 400 through 420 nm such as in a blue-violet semiconductor laser and blue-violet SHG laser for converting the wavelength of an infrared semiconductor laser using the second harmonic wave. When employing an objective lens having the same numerical aperture (NA) as that of the DVD (Digital Versatile Disc), use of such a blue-violet laser light source allows recording of 15 through 20 GB information on an optical disc having a diameter of 12 cm. When the NA of the objective lens is increased to 0.85, it permits recording of 23 through 25 GB information on the optical disc having a diameter of 12 cm. In the following description of this specification, an optical disc using a blue-violet laser light source and a photomagnetic disc will be collectively called the high-density optical disc.
In the high-density optical disc employing an objective lens having the NA of 0.85, there is an increase in the comatic aberration caused by the skew of an optical disc. Accordingly, in some cases, the protective layer is designed to be thinner than that of a DVD (0.1 mm as compared to 0.6 mm for the DVD), thereby reducing the comatic aberration resulting from the skew. Incidentally, the value of the optical disc player/recorder as a commercial product cannot be made sufficient merely by the capability of providing appropriate recording/reproduction of information using the high-density optical disc of this type. In view of the fact that a DVD and CD (Compact Disc) carrying a great variety of information are currently placed on the market, the capability of ensuring appropriate recording/reproduction of information using the high-density optical disc is not sufficient. For example, appropriate recording/reproduction with the DVD and CD owned by a user will enhance the commercial value of an optical disc player/recorder for high-density optical disc. Thus, the optical pickup apparatus mounted on the optical disc player/recorder for high-density optical disc is required to provide the function of appropriate recording/reproduction of information using any one of the high-density optical disc, DVD and CD in an interchangeable manner.
One of the methods for ensuring appropriate recording/reproduction of information using any one of the high-density optical disc, DVD and CD in an interchangeable manner is selective switching among the optical systems for high-density optical disc, DVD and CD, in response to the recording density of the optical disc for recording and reproducing information. However, this method requires use of a plurality of optical systems, and is not suited for product downsizing and cost cutdown.
Thus, in order to simplify the arrangement of an optical pickup apparatus characterized by medium interchangeability and to reduce the cost, it is preferred to promote compatibility between the optical system for high-density optical disc and those for the DVD and CD, and to minimize the number of optical components constituting the optical pickup apparatus. Promoting compatibility between the objective optical systems arranged opposite to the optical disc brings an advantage in simplifying the optical pickup apparatus and reducing the cost. To get a standardized objective optical system compatible with various types of optical discs having different wavelengths for recording/reproduction, a phase structure characterized by dependency of spherical aberration on wavelength must be incorporated in the objective optical system.
The European Provisional Publication No. 1304689 discloses an objective optical system, having a diffraction structure as a phase structure, compatible with a high-density optical disc and conventional DVD and CD, and an optical pickup apparatus carrying this objective optical system.
However, the objective optical system disclosed in the Patent Document 1 has a big difference in magnification for recording/reproduction of information using each optical disc. In the optical pickup apparatus, this makes it difficult to standardize the optical components other than the objective optical system or to use the light source module where multiple types of light sources are integrated. For this reason, simplification of the configuration of the optical pickup apparatus or cost reduction cannot be achieved. This has created a problem in prior art. Another problem in the prior art is found in the increased comatic aberration at the time of lens tracking, due to a large magnification for recording/reproduction of information with the CD in particular.